Hypoxic cells in tumours are resistant to ionising radiation, and are a major cause of treatment failure in radiation therapy (Movsas et al., Cancer 2000, 89, 2018; Rudaf et al., Radiother. Oncol. 2000, 57, 31). Hypoxic cells are also considered to compromise response of solid tumours to cytotoxic chemotherapy (Brown and Giaccia, Cancer Res. 1998, 58, 1408). The 1,2,4-benzotriazine di-N-oxide tirapazamine (TPZ) is selectively toxic to hypoxic cells because of its metabolic activation to a cytotoxic species by one-electron reduction (Baker et al., Cancer Res. 1988, 48, 5947; Laderoute et al., Biochem. Pharmacol. 1988, 37, 1487; Brown, Br. J. Cancer 1993, 67, 1163). As shown below, the initial one-electron reduction product TPZ* is reoxidised to the starting compound by dioxygen, thereby preventing cytotoxicity in oxic cells.
TPZ is therefore of interest for killing hypoxic cells in tumours, thereby improving overall response during radiation therapy. TPZ also has potential for combination with standard cytotoxic chemotherapy (Dorie and Brown, Cancer Res. 1993, 53, 4633; Langmuir et al., Cancer Res. 1994, 54, 2845; Dorie and Brown, Cancer Chemother. Pharmacol. 1997, 39, 361), with (at least) two mechanisms of therapeutic synergy. The first mechanism is the killing of resistant hypoxic cells (analogous to the mechanism of interaction with radiotherapy), and the second is the interference with repair of chemotherapy-induced DNA damage in hypoxic cells as has been demonstrated for cisplatin (Kovacs et al., Br. J. Cancer 1999, 80, 1245; Peters et al., Cancer Res. 2001, 61, 5425).
TPZ has already demonstrated significant antitumour activity in early phase human clinical trials in combination with ionising radiation and/or cisplatin chemotherapy (for a review, see Denny and Wilson, Exp. Opin. Invest. Drugs 2000, 9, 2889), and a multicentre phase III trial of TPZ in combination with cisplatin and radiation for treatment of head and neck tumours is in progress (Rischin et al., J. Clin. Oncol. 2005, 23, 79-87). While TPZ shows promising indications of clinical activity, it also displays considerable toxicity, such as neutropenia, thrombocytopenia, nausea, vomiting, diarrhea and muscle cramping. These toxicity limitations preclude administration of doses that are either high enough or sufficient enough to exploit hypoxia fully during cancer treatment. Although the mechanisms of TPZ toxicity towards normal tissues are not fully understood, it is considered that the toxicity arises at least in part because of redox cycling (Elwell et al., Biochem. Pharmacol. 1997, 54, 249; Wouters et al., Cancer Res. 2001, 61, 145), and is therefore considered to be distinct from the mechanism of hypoxic cell killing.
There have been only limited structure-activity studies on analogues of TPZ. Kelson et al (Anti-Cancer Drug Design 1998, 13, 575-592) disclosed compounds of type A, where X was H or an electron-withdrawing group, n was 2 or 3, and R was Me or Et. The main conclusion from this paper was that compounds with dialkylaminoalkyl side chains showed decreased hypoxic selectivity in vitro and comparable but not superior activity to TPZ in vivo. There was no clear relationship between the electron-withdrawing capability of the 7-substituent on the benzo ring and biological activity. Hay and Denny (Tet. Lett. 2002, 43, 9569-9571) and Kelson et al (Anti-Cancer Drug Design 1998, 13, 575-592) described compounds of type B, where X is H or hydroxyalkyl, n is 2 or 3, and R is OH or OMe, but did not describe any biological activity. Finally, Hay et al. (J. Med. Chem. 2003, 46, 169-182) showed, for compounds of type C, where X is NEt2, NMe2, OMe, Me, Cl, F, CF3, MeSO2, nBuSO2, and NO2, that oxic cytotoxicity in SCCVII cells in vitro correlated with one-electron reduction potential [E(1)], but there was not a clear relationship between in vitro hypoxic cytotoxicity and E(1). Further, there was no clear relationship between hypoxia-selectivity and E(1) and none of the compounds displayed improved in vitro activity compared to TPZ.

To a large extent the above efforts to identify analogues of TPZ with improved therapeutic activity have focused on compounds with higher reduction potentials, with the expectation that such compounds will be metabolically activated more rapidly than TPZ under hypoxic conditions and will therefore have improved activity against hypoxic cells in tumours.
In the present invention the inventors have unexpectedly found that certain tricyclic triazine compounds of the invention have activity against hypoxic tumor cells in vivo despite having lower reduction potentials than the corresponding compounds in the literature (Kelson et al, Anti-Cancer Drug Design 1998, 13, 575-592).
It is an object of the present invention to provide a range of novel tricyclic 1,2,4-triazine-1-oxides and novel tricyclic 1,2,4-triazine-1,4-dioxides and their related analogues, and to provide for their use as potentiators of the cytotoxicity of anticancer drugs and as radiosensitizers and as hypoxia-selective cytotoxins for cancer therapy in combination with radiation and/or with other anticancer agents, or to at least provide the public with a useful choice.